
In 1976, an estimated 25,000 runners finished marathons in the United States by 2016, the estimated number of competitors who completed a 26.2-mile course had soared to 507,000. The Boston Marathon, which kicked off in 1897 and is the world’s oldest annual marathon, began allowing female competitors in 1972, while the first Olympic marathon for women wasn’t held until 1984. For decades, marathons were only open to male athletes. In America alone, there are now more than 1,100 marathons each year. Today, marathon races take place everywhere from the North Pole to the Great Wall of China. The random boost in mileage ended up sticking, and in 1921 the length of a marathon was formally standardized at 26.2 miles (42.195 kilometers). If you can complete 3 miles, you can start to train for a half-marathon. It's a good goal for runners who have already completed a 5K (3.1 mile) race or a 10K (6.2 mile) race and are looking for a new challenge. This is exactly half the distance of a full marathon (26.2 miles). As the story goes, Queen Alexandra requested that the race start on the lawn of Windsor Castle (so the littlest royals could watch from the window of their nursery, according to some accounts) and finish in front of the royal box at the Olympic stadium-a distance that happened to be 26.2 miles (26 miles and 385 yards). A half-marathon is 13.1 miles or 21 kilometers.

For the next few Olympics, the length of the marathon remained close to 25 miles, but at the 1908 Games in London, the course was extended, allegedly to accommodate the British royal family.
